Patent GB-1204179-A (GB)
Title: Beat frequency sonic technique and apparatus for use in seismic surveys
Inventor: BODINE ALBERT GEORGE JR
IPC: G01V1/047
Filed: 1967-10-18
Abstract: 1,204,179. Vibrators. A. G. BODINE. 18 Oct., 1967 [20 Oct., 1966], No. 47485/67. Heading F2X. [Also in Division H4] In an arrangement for generating seismic energy the outputs of separate mechanical generators producing sonic energy at two different frequencies are coupled by mechanical coupler means to a mechanical mixing device coupled to the ground, the mixing device producing sonic energy at the difference frequency. By varying the frequency of one of the generators the difference frequency may be swept over a range of frequencies. In all of the embodiments the generators comprise "orbiting-mass" oscillators and the coupler means comprise resonant bar members having high "Q" sonic vibrational characteristics such as steel. Figs. 2 and 4 show one embodiment in which a first coupler means 13 is coupled to a pair of "orbiting-mass" oscillators 12a, 12b driven at a first frequency by an electric motor 50 and a second pair of coupler means 14a, 14b connected to respective "orbiting-mass" oscillators 11a, 11b driven at a second frequency by an electric motor 60 and belt drive 65. The oscillators are so phased that horizontal vibrations cancel out and the vertical vibrations are then coupled to a heavy pedestal member 28 (e.g. of metal) either directly (e.g. by a press-fit, Fig. 5, not shown), or, as shown, by resilient means such as rubber bushings 35 which give a certain amount of pre-mixing. The vibrations of member 28 are then coupled to the ground by mixing member 27 comprising material which exhibits non-linear response characteristics to applied forces e.g. rubber or urethane. The vibration frequency of the coupler means is of the order of 500 c.h.s. the frequency of either bar 13 or of bars 14a and 14b being made 20-60 cycles higher so that by varying the speed of either motor 50 or 60 over a range of about 40 c.h.s. a corresponding variation is produced in the output of the mixing member 27. In another embodiment sonic energy produced by respective generators comprising two pairs of "orbiting-mass" oscillators 108a, 108b and 110a, 110b is coupled by respective tubular members 101, 102 to the mixing member 100 at threaded regions 100a and 100b respectively. In operation member 100 is driven into the ground and an hydraulically operated member 122 is driven downwardly so that it expands a clamping portion 100c which couples the energy into the ground in which the mixing action is effected. In a modification of the latter embodiment the two coupling members instead of being arranged coaxially, as in Fig. 8, are inserted in the groundinaside-by-sideparallel arrangement (Figs. 10-12, not shown). In a further arrangement the "orbiting-mass" oscillators 11, 12 (Fig. 13) are connected via respective coupler means 14, 13 to members 141, 142 (comprising e.g. two halves of a cone) inserted in the ground and separated by cylindrical roller members 143 which allow free relative motion of the members. The construction of the "orbiting-mass" oscillators is described with reference to Fig. 7 (not shown).